SequoiaView: Shows you the size of the files and directories on your HDD in a way that you can find the largest ones almost instantly.

IrfanView: Can display almost any image format and has a small footprint.

StrokeIt: Just like OptiMoz brings gesture recognition to Firefox, this little tool gives you gesture command abilities to the Windows Desktop in general. (Just don't forget to exclude the Firefox-window from recognition, if you install both!).

For developers, there are a few extra packages:

TortoiseCVS: An extension to the Windows Explorer, which integrates CVS directly into the right-click dropdown menu.

Synergy: Enables seamless mouse- and keyboard sharing across several computers/screens and even between different platforms. Also synchronizes the clipboard.